1. Field of the Invention
The present invention relates generally to column stabilized floating structures and more particularly to a floating oil and gas drilling and/or production platform having a minimum response to the excitation imparted thereon by a seaway.
2. Description of the Prior Art
As the search for offshore oil proceeds to deeper water, the cost of finding and producing oil and gas becomes very expensive. The primary cost is that of the platform structure which supports the production equipment and/or the wellhead equipment through which the oil is produced.
For a platform to be fixed to the seabed in deep water, the support structure part would be very expensive but the wellhead system part would be relatively inexpensive. For present day relatively shallow floating production systems, the supporting structure part is relatively inexpensive, but due to motions in a seaway, the wellheads (known as "wellhead trees") must be placed on or near to the wellbores in the seabed and remote from operating personnel, which requires that the wellhead equipment be very intricate, elaborate, and consequently very expensive and relatively inefficient.
Also, when a severe storm approaches the production site, due to the excessive motions of the floating structure, it is common practice to disconnect the elaborate subsea wellhead equipment. This disconnection and the subsequent reconnection, after the severe storm passes, impose considerable down time which results in lost production and consequently a loss of revenue.
Therefore, it is desired (1) to have a free-floating, economical production platform unit with very low motion response, particularly minimum heave, so that the wellheads can be installed onboard the platform where they would be readily accessible for operation and maintenance, and (2) to eliminate the need to disconnect the wellhead equipment between the seabed and the platform when a severe storm occurs, see U.S. Pat. Nos. 4,850,744, 4,934,870, 4,936,710 and 4,913,592, assigned to the same assignee.
The problems caused for a floating platform by excessive motions including heave are well described in the patent literature, see for example, U.S. Pat. Nos. 4,112,864 and 4,167,147.
It is the primary object of the present invention to reduce the undesirable heave by (1) utilizing the positive effects of the water plane provided by the columns and (2) by reducing the hydrodynamic forces acting on the submerged lower hull, thereby allowing oil and gas production from surface type wellheads and associated equipment to be suspended from the floating platform unit, instead of being on or near the seabed.
Further objects of this invention are to provide a floating production unit which has extremely low vertical and angular motion responses to wind and waves, which is capable of withstanding severe storms without the need to disconnect the onboard wellhead equipment between the floating structure and the seabed, and which is economical to manufacture, competitively priced with, and more efficient than known types of offshore production structures.